The quality and performance of a digital transmitter is determined by the distance over which the transmitted digital signal can propagate without severe distortions. This is typically characterized as the distance over which a dispersion penalty reaches a level of ˜1 dB. A standard 10 Gb/s optical digital transmitter, such as an externally modulated optical source (e.g., a laser), can transmit up to a distance of ˜50 km in standard single mode fiber, at 1550 nm, before the dispersion penalty reaches the level of ˜1 dB. This distance is typically called the dispersion limit. The dispersion limit is determined by the fundamental assumption that the digital signal is transform-limited, i.e., the signal has no time-varying phase across its bits and the signal has a bit period of 100 ps, or 1/(bit rate) for a 10 Gb/s optical digital transmitter.
Three types of optical transmitters are currently in use in prior art fiber optic transmission systems:
(i) Directly Modulated Laser (DML) transmitters;
(ii) Electroabsorption Modulated Laser (EML) transmitters; and
(iii) externally modulated Mach Zhender (MZ) transmitters.
For transmissions in standard single mode fiber, at 10 Gb/s and 1550 nm, it has generally been assumed that MZ and EML transmitters can achieve the longest reach, typically reaching 80 km. Using a special coding scheme, generally referred to as phase shaped duobinary transmission, MZ transmitters can reach 200 km. On the other hand, DML transmitters generally reach <5 km because their inherent time dependent chirp causes severe distortion of the signal after this <5 km distance.
Azna LLC of Wilmington, Massachusetts has recently developed a novel line of CML transmitters which can exceed the aforementioned <5 km transmission limit of conventional DML transmitters. By way of example but not limitation, various novel DML systems for long-reach optical data transmission (e.g., >80 km at 10 Gb/s) through optical fibers in single mode fiber are disclosed in the eight (8) above-identified U.S. patent applications (which patent applications are hereby incorporated herein by reference). The CML transmitter associated with these novel systems is sometimes referred to by Azna LLC as a Chirp Managed Laser (CML)™. In these new CML systems, a Frequency Modulated (FM) source (e.g., a laser) and an optical spectrum reshaper (e.g., a filter) which uses the frequency modulation to increase the amplitude modulated signal and compensate for dispersion in the transmission fiber. In one novel CML embodiment, the frequency modulated source may comprise a Directly Modulated Laser (DML). The Optical Spectrum Reshaper (OSR), sometimes referred to as a frequency discriminator, can be formed by an appropriate optical element that has a wavelength-dependent transmission function (e.g., a filter). The OSR can be adapted to convert frequency modulation to amplitude modulation.
A wavelength tunable laser source is of great interest in Wavelength Division Multiplexing (WDM) systems in which a number of different laser wavelengths are used to simultaneously transmit multiple channels of digital information through an optical fiber. This is because a single wavelength tunable laser can be used to generate the light signal needed for any one of the different channels, thereby greatly simplifying equipment maintenance and inventory considerations.
In the prior art, tunable lasers are typically externally modulated devices which use lithium niobate or InP Mach Zehnder modulators. The signals generated by these externally modulated optical transmitters require a relatively high drive voltage. In addition, the dispersion distance of these external modulators can be limited: for example, these external modulators typically have a dispersion limit of <80 km, at 10 Gbps at 1550 nm, in standard Single Mode Fiber (SMF).